1. Field of the Invention
This invention relates generally to semiconductor processing equipment and more particularly to ion implantation end stations.
2. Description of the Related Art
Ion implanters are used to implant various types of ions into semiconductor wafers to change the electrical characteristics of the wafers. For example, boron, phosphorous, arsenic, and oxygen are all typically implanted to create p-type, n-type, or insulating regions within the semiconductor wafer.
In an ion implantation machine, an ion beam is generated and is scanned across the surfaces of the wafers. During this process, the beam also impinges upon surrounding surfaces, such as the wafer support wheel, causing ion implantation into these surfaces.
The problem of cross-contamination arises when ions of different types are implanted using the same ion implanter apparatus. For example, if wafers are first subjected to a phosphorous implant a certain percentage of the phosphorous ions will be implanted into the surrounding surfaces. If another set of wafers are then subjected to an arsenic implant some of the phosphorous ions which were implanted into the surrounding surfaces will be released, contaminating this new set of wafers with phosphorous.
This problem has been addressed in the prior art in several ways. One obvious, but expensive, solution is to have a dedicated ion implanter for each species of ions to be implanted thereby eliminating any chance of cross contamination. However, with ion implanters costing in the range of $500,000-$2,000,000 this is not a particularly desirable solution.
Another solution to the cross-contamination problem is to remove the contaminated surfaces by either thoroughly cleaning them with an abrasive substance, or by re-machining them. However, this is an expensive process which results in considerable machine downtime. Furthermore, the cleaning or re-machining process can introduce undesirable particulates into the ion implantation machine.
A partial solution to the cross-contamination problem is to minimize the surrounding surface area into which ions can be implanted. This approach is taken with the Precision 9000 and Precision 9200 ion implanters produced by the Applied Implant Technology Division (Horsham, England) of Applied Materials, Inc. (Santa Clara, Calif.). In the Precision 9000 and 9200 systems, wafers are supported on a spoked wheel thereby presenting minimal surface areas for potentially cross-contaminating implantation into surrounding surfaces. However, even the Precision 9000 and 9200 systems present the potential for cross-contamination due to implantation into the spokes of the wafer support wheel.